Lubricating oil base stocks that are used to prepare lubricating oils are highly colored and are unstable in the presence of oxidizing agents and UV light. Upon exposure of the lubricating oil or the base stock to an oxidizing agent or to UV light, a precipitate may form which affects the quality of the product. Lubricating oil base stocks that have been recovered as distillates from a hydrocracking zone in particular contain olefin and aromatic components that impart poor stability to the lubricating oil. Hydrogenation of the lubricating oil base stock, often referred to in the art as hydrofinishing, is a known method for improving the stability of lubricating oils.
U.S. Pat. No. 3,852,207 to Stangeland et al., discloses a catalytic hydrogenation process for lubricating oil which involves a hydrogenation catalyst comprising a refractory oxide and a platinum group metal.
U.S. Pat. No. 3,962,071 to Itoh et al., claims a process which includes hydrocracking, fractionation, and hydrogenation for enhancing lubricating oil photostability. In Itoh et al., the hydrogenation catalyst includes palladium on a silica containing refractory inorganic oxide carrier having 5-40 weight percent silica, a surface area of 100-500 m.sup.2 /g, a pore volume of 0.5-1.2 ml/g, an average pore diameter of 30-120 Angstroms, and a bulk density of 0.5-0.7 g/ml.
U.S. Pat. No. 3,637,484, U.S. Pat. No. 3,637,878, and U.S. Pat. No. 3,703,461, all to Hansford, disclose a process for hydrogenating an aromatic hydrocarbon feedstock with a catalyst having a support composed of a silica-alumina cogel in a large pore alumina gel matrix and containing a platinum group metal. The catalyst composition of Hansford has a pore volume of 0.8-2.0 ml/g with about 0.3-1.0 ml/g of the pore volume in pores of diameter greater than about 500 Angstroms. In Hansford, the support employs a relatively high ratio of alumina to silica, always in excess of 60 weight percent of alumina as compared to silica. Conversely, supports having high ratios of silica to alumina have been disclosed in U.S. Pat. Nos. 4,139,493; 4,325,805; and 4,601,996. However, catalysts containing higher ratios of silica to alumina generally contain a hydrogenation component other than a platinum group metal, commonly nickel, tin, molybdenum, or cobalt.
Nevertheless, the hydrogenation processes available in the art for stabilizing lubricating oil base stocks do not adequately address the problem of catalyst fouling. Producing base stocks of high stability requires removing unstable hydrocarbon molecules to very low levels. A hydrogenation process employing a single hydrogenation catalyst will not easily accomplish this while maintaining acceptable catalyst activity. We have now discovered a unique layered catalyst system having a low rate of catalyst deactivation and a high activity when used for stabilizing a lubricating oil base stock.